Antennas are widely used throughout the industrialized world to transmit and receive electromagnetic energy. The electromagnetic energy is typically used to carry some sort of signal which can be decoded to result in usable data. Examples of the uses of antennas include cellular telephones, television, radio, radar, and numerous other applications.
An exemplary antenna of the prior art is depicted in FIG. 1. Antenna 100 contains a reflector 102 and a feed 104. The assembly containing reflector 102 and feed 104 is typically supported by some form of stand or structure 106. Antenna 100 operates in the following manner: In receiving mode, reflector 102 reflects electromagnetic waves and directs the electromagnetic waves to feed 104. In transmission mode, feed 104 transmits an electromagnetic wave that is reflected to the environment via reflector 102. The signal from/to feed 104 passes through a sutiable transmitter/receiver 108.
Another view of an antenna is presented in FIG. 3. Parabolic reflector 302 performs substantially the same function as reflector 102 of FIG. 1, i.e., directing electromagnetic waves to and from the correct point. Feed 310 contains a reflector element 304 and a dipole 306. Reflector element 304 directs the signal to/from parabolic reflector 302 to dipole 306. Dipole 306 is typically coupled to a transmitter or receiver (not shown), possibly through a device to amplify the signal.
When using antennas for communication purposes, it is often desirable to restrict the electromagnetic signal being transmitted/received to within a certain frequency range. Different frequencies are used for different purposes. For example, cellular phones are assigned frequencies within a particular range, AM and FM radio stations are assigned frequencies within another range, and so forth. When an electromagnetic signal contains frequencies that are not desirable, it becomes difficult to separate the correct signal from the unwanted frequencies (i.e., “noise”). Thus, it is desirable to maximize the signal-to-noise ratio, which is expressed in decibels (dB).
In the prior art, many antenna systems incorporate a bandpass filter between the antenna and the receiver or between the antenna and the transmitter. The effect of a bandpass filter is illustrated in FIGS. 2A and 2B. FIG. 2A presents a signal without the use of a bandpass filter. The X-axis 202 is the frequency of the signal and the Y-axis 204 is the amplitude of signal 206. It is evident that there is a large amount of signal that is outside of the desired center frequency 208. FIG. 2B presents the signal after being passed through a bandpass filter. It can be seen that signals outside of a particular range are at a lower amplitude than the signals at the desired frequency 208. By reducing the signal outside of the desired frequency, it is easier for transmitter/receiver 108 to process the signal because of the reduction in noise. Furthermore, it should also be noted that by reducing the unwanted frequencies during transmit, there is less noise introduced into the atmosphere, thereby improving the performance of other systems operating in the same frequency range, even though those systems may not have bandpass filtering.
In the prior art, the bandpass filter is typically a four or six pole ceramic filter. These filters are effective at removing unwanted frequencies, but are undesirable in a number of respects. For example, the cost of such ceramic filters can be very high. Furthermore, a significant insertion loss (a reduction of signal strength of the processed signal compared to the unprocessed signal) may be introduced by these ceramic filters, in some cases greater than 0.4 dB. Because of the low strength of the received signal, such an insertion loss is highly undesirable.